1. Technical Field
The present disclosure relates to a power amplification device and a transmitter that use a plurality of class-D power amplifiers to amplify transmission signals.
2. Description of the Related Art
Typically, power amplification devices provided in transmitters used for wireless communication amplify very weak amplitudes of input signals and output the resulting signals in order to obtain signal output strengths necessary for wireless systems. Since the very weak amplitudes of input signals are amplified to obtain signals with large output power, power consumed in wireless blocks in the power amplification devices is large. The power consumption has a large influence on, particularly, the drive times of battery-powered wireless devices, such as mobile phones. Accordingly, the power amplification devices require high power efficiencies.
One possible means for enhancing the power efficiencies is using class-D power amplifiers. Class-D power amplifiers utilize saturation operations of transistors, and ideally, electrical current flows only in a switching period, and no unwanted electrical current flows, thus making it possible to obtain high power efficiencies.
In recent years, an orthogonal frequency-division multiplexing (hereinafter “OFDM”) system used for wireless local area networks (LANs) is used as a modulation system in order to enhance the spectral efficiency. The OFDM system is a system in which carrier waves having different frequencies are modulated and multiplexed, and the carrier waves are orthogonal to each other. The OFDM system has a high power peak relative to an average power at a timing when the phases of carrier waves overlap each other. The ratio of a peak power to an average power is represented by a peak-to-average power ratio (hereafter “PAPR”) and may be about 10 dB in the OFDM system. In the OFDM system, the PAPR is large in principle, and a linear amplifier is required in order to suppress an influence of, for example, inter-symbol interference caused by distortion. When the peak power is set to the saturation power of a power amplifier, the average power takes a small value relative to the saturation power. In this case, since the power amplifier cannot be operated at an operating point at which the power efficiency is high, the power efficiency during output of the average power decreases. Herein, the difference between the maximum power and the average power is referred to as an “amount of back-off”. The power amplifier operates at an operating point at which the power efficiency is lower, as the amount of back-off increases.
One possible means for overcoming such a problem is using a switched-capacitor power amplifier. In a switched-capacitor power amplifier, the number of, among a plurality of small-size amplifier cells, amplifier cells that are to operate is controlled using a digital code, thus making it possible to linearly control an output voltage of the switched-capacitor power amplifier. Such a switched-capacitor power amplifier is described in “A Switched-capacitor RF Power Amplifier Solid-State Circuits, IEEE Journal of Volume 46, Issue 12 pp. 2977-2987, December 2011”. Switched-capacitor power amplifiers can be said to be radio-frequency digital-to-analog converters (RF-DACs) that output high-frequency signals having amplitudes controlled with digital codes, and have a problem in obtaining favorable linearity. In the following description, the digital codes with which the amplitudes are controlled are referred to as “AM codes”.
One means for overcoming such a problem with the switched-capacitor power amplifiers is a digital-to-analog (D/A) conversion circuit disclosed in Japanese Unexamined Patent Application Publication No. 2012-175440. The D/A conversion circuit disclosed in Japanese Unexamined Patent Application Publication No. 2012-175440 ensures linearity by dynamically varying capacitance assignment to individual bits in input data.
In the switched-capacitor power amplifiers of the related art, however, even when the accuracy of capacitance values is adjusted, as in Japanese Unexamined Patent Application Publication No. 2012-175440, there is a problem in that the linearity cannot be sufficiently improved, since the accuracy of resistance values is not adjusted. In particular, in circuits of switched-capacitor power amplifiers, it is necessary to pass large electrical current in order to output signals with large power. When large electrical current is passed through a wire that provides connection between elements, an influence of a voltage drop in the wire increases, which causes a problem of linearity deterioration due to the accuracy factor of a resistance value.